A model of world populations is being derived. It includes a theory of the course of evolution which sees enhancement of awareness and responsiveness as the major central tendency. Preservation of this central tendency requires a gradual shift from an increasing to a decreasing human population. The change between these two trends has just begun. A maximum of more than eight billion people will be reached before decline in numbers begins. Certain traps or dangers will exist during the transition toward a pattern of decline in numbers. One of these traps results from an aging population: Our modeling from mouse and rat population studies indicates that a high probability of population extinction may occur when over 50 percent of the population consists of postreproductive individuals. The broader interpretation of this model holds that excessive numbers of postreproductive or postproductive individuals inhibit acquistion by younger members of those social roles and other behaviors essential for species survival.